1. Field of the Invention
The present invention relates to an apparatus and a method for inspection of a mask blank suitable for, for example, EUVL (Extreme Ultra Violet Lithography) using extreme ultraviolet radiation with a wavelength of about 13.5 nm. The invention also relates to a method for manufacturing a reflective exposure mask, a method for reflective exposure, as well as a method for manufacturing semiconductor integrated circuits.
2. Description of the Related Art
Semiconductor devices, such as semiconductor integrated circuits, are mass produced by repetitively using a optical lithography process in which a mask, i.e., a master having a circuit pattern drawn thereon, is irradiated with exposure light so that the pattern is transferred onto a semiconductor substrate (hereinafter, referred to as “wafer”) via reduction projection optics.
In recent years, as scale-down of semiconductor devices have been progressing, there are discussed methods for enhancing the resolution by further shortening the exposure wavelength of optical lithography. While ArF lithography using argon fluoride excimer laser light having a wavelength of 193 nm has been developed so far, EUVL having a far shorter wavelength of 13.5 nm has been being developed.
In the EUV wavelength region, since transmissive masks cannot be used in terms of light absorption by substances, multilayered reflective substrates, which can effect reflection due to a multilayer film of, e.g., Mo (molybdenum) and Si (silicon) (i.e., Bragg reflection), are employed for mask blanks for EUVL. The multilayer-film reflection is a reflection exploiting a type of interference. In a mask for EUVL, an absorber pattern is formed on a multilayer coated mask blank which has a multilayer film of, e.g., Mo and Si deposited on quartz glass or low thermal expansion glass substrate.
In EUVL, because of reflective masks using Bragg reflection and an extreme short exposure wavelength of 13.5 nm, occurrence of even slight height irregularities as small as a fraction of the exposure wavelength may cause local differences in reflectivity due to those height irregularities, resulting in defects during transfer processes. Accordingly, masks for EUVL largely are different in quality of defect transfer from conventional transmission masks.
For the mask blank defect inspection in the preceding step prior to formation of an absorber pattern, two methods are available: one is a method in which a mask blank is obliquely irradiated with laser light to detect any foreign object from its diffused reflection light, and the other is an at-wavelength defect inspection method in which EUV light of the same wavelength as that for use in makes pattern exposure is used for defect detection. The latter method further includes a method employing dark field images (see, e.g., JP-2003-114200 A), an X-ray microscope method employing the bright field images (see, e.g., JP-6-349715 A (1994)), and a dark-field bright-field combinational method in which dark field images are used for defect detection and then defect identification is performed in the bright field system using a Fresnel zone plate (see, e.g., US 2004/0057107 A).
Incidentally, for conventional transmissive mask blank inspections, two methods are known: a mask blank is obliquely irradiated with laser light to detect any foreign object from its diffused reflection light in one method, and a bright field image (microscopic image) is detected in another method. Modifications of the latter method is to discriminate between convex defects and concave defects based on asymmetries of detected image signals (see, e.g., JP-2001-174415 A, and JP-2002-333313 A).
Further, yet another method is disclosed in which a peelable pattern is formed on a multilayer coated mask blank, and then actual pattern is transferred therewith, and then the pattern is examined to inspect multilayer-film defects (see, e.g., JP-11-354404 A (1999)).
However, in JP-2003-114200 A, the dark field detection method employing EUV light is highly sensitive in detection and excellent in detection performance for phase defects due to irregularities of multilayer film, but incapable of discriminating between concave and convex defects simultaneously.
Also, in JP-6-349715 A (1994), the X-ray microscope method employing the bright field examines only the reflection ratio of the multilayer film, hence, all of defects causing changes in phase cannot be detected.
Also, in US 2004/0057107 A, the method, that is an exposure wavelength inspection serving as both a bright field inspection and a dark field inspection, involves more complicated inspection equipment and, although being a high-speed dark field inspection, yet is not highly sensitive in detection.
Also, as in JP-2001-174415 A, and JP-2002-333313 A, the method employing laser is insufficient in sensitivity because defects to be detected are too small as compared with the inspection wavelength. Moreover, the method can detect concave and convex defects residing only on the surface of the multilayer film, but cannot capture defects which reside inside the multilayer film and may cause abnormalities of EUV light reflection.
Further, in JP-11-354404 A (1999), the method in which a peelable pattern is formed on the multilayer coated mask blank, and then actual pattern is transferred therewith, and then the pattern is examined to detect multilayer-film defects, can detect phase defects, but requires a further step of actual transfer of pattern transfer, resulting in cumbersome inspection.
In any of the inspection methods as described above, in case any defect that is hard to repair is detected, even if the defect is of minute size, the mask blank involved is regarded as a defective, being put into disposal.